1. Field of the Invention
This invention relates to techniques for direct conversion of hydrocarbons from a gaseous form to a liquid form. More particularly, the invention relates to methods and apparatus for reactive conversion of hydrocarbons, such as direct natural gas to liquid conversion.
2. Background Technology
Methane is an abundant hydrocarbon fuel and chemical feed stock, and is expected to remain so for quite some time. Yet, because of capital and technological barriers, methane has remained an under-utilized resource throughout the world. It is desirable to upgrade available methane to methyl or higher oxygen atom containing hydrocarbons, such as alcohols, ethers, aldehydes, etc. Existing technologies for converting methane to methanol include destruction of methane to form a synthesis gas (H2 and CO), followed by indirect liquefaction steps.
Conventional catalytic approaches to produce methanol from methane typically have poor conversion efficiencies, slow reaction rates, and are not economically competitive because they are typically so energy intensive. One such process, the oxidative coupling process, involves the use of oxidants to abstract hydrogen from methane, and coupling two or more hydrocarbon radicals to form light olefins, oxygenates, and other hydrocarbons. The oxidants are oxygen, halogens and reducible metal oxides as oxygen carriers and catalysts. In the oxidative coupling process, hydrogen abstraction at the oxygen centers of the catalyst is typically the rate-determining step, and catalyst properties are important for end product selectivity. Therefore, the maximum rate of product conversion strongly depends on the rate of radical formation on the active oxygen centers. In order to increase the rates, chemists have used high temperatures, even in excess of 900° C. However, this undesirably promotes deep oxidation of methane to fully oxidized species, such as CO2.
In another approach, a high temperature dehydrogenation coupling process is employed that has a very high radical generation rate, and correspondingly a high rate of light olefin formation. However, this process is plagued by solid carbon formation which lowers the efficiency of the olefin production, and excess hydrogen is necessary to suppress the solid carbon formation.
In U.S. Pat. No. 5,427,747 to Kong et al. (hereinafter “Kong”), the disclosure of which is incorporated herein by reference, a method for producing oxygenates from hydrocarbons is described that utilizes a chemical reactor for oxygenating hydrocarbons. The chemical reactor includes a dielectric barrier discharge plasma cell which includes a pair of electrodes having a dielectric material and void therebetween, and a hydrocarbon gas inlet feeding to the void. The reactor also has a solid oxide electrochemical cell (SOEC) that includes a solid oxide electrolyte positioned between a porous cathode and a porous anode, and an oxygen containing gas inlet stream feeding to the porous cathode side of the electrochemical cell. A first gas passageway feeds from the void to the anode side of the electrochemical cell. A gas outlet feeds from the anode side of the electrochemical cell to expel reaction products from the chemical reactor.
In another technique for gas to liquid conversion, an apparatus is employed that uses a high temperature ionic conducting electrolyte membrane plate for oxygen anion diffusion. A porous cathode and a porous anode are attached on opposite surfaces of the electrolyte plate. An inert ceramic tube is bonded to the electrolyte plate to form an SOEC cell structure. The SOEC cell must use an external power source for operation.
Other approaches include the so-called Fischer-Tropsch and other indirect processes for liquid production from natural gas. These processes rely on steam reforming or partial oxidation of natural gas to synthesis gas, and use high temperatures, high pressures, and catalysis. The Fischer-Tropsch processes have the disadvantages of being capital and energy intensive, having low overall production yield of liquid, and requiring multiple passes to get a desirable liquid yield.
Accordingly, it would be desirable to provide improved apparatus and methods for converting gas to liquids that avoids or overcomes the difficulties and problems of prior techniques.